Membrane Gas Separation

3.4
Conclusions 
PTMSN, described in this work, is another member of a family of high permeability, 
large free volume membrane materials. It is characterized by relatively high gas perme-
ability caused mainly by great solubility coeffi cients, small or negative activation energies 
Table 3.10  Sizes of free volume elements in polymers as determined by the PALS and 
 IGC methods [19,35]
a

Polymer
P (O 
2
), Barrer
V
f
( Å
3
)
PALS
IGC
V
c
V
b

b

PVTMS
44
345
±
10
610
235
AF1600
170
490
±
10
710
270
AF2400
1140
880
±
20
710/1100
2700/455
PTMSN
800 – 1000
775
710/1260
270/500
PTMSP
7000 – 12 000
1320
±
35
–
–
a
AF1600 and AF2400 are random copolymers of 2,2 - bis(trifl uoromethyl) - 4,5 - difl uoro - 1,3 - dioxole and 
tetrafl uoroethylene with content of the latter of 35 and 13 mol%, respectively.
b
V
b
were calculated via Benson formula V
b
= V
c
/(0.422 log p
c
+ 1.981), where p
c
is the critical pressure.

---

56
Membrane Gas Separation
of permeation, solubility controlled permeation that is revealed by increase in permeabil-
ity coeffi cients when the size of the penetrants increases, and great free volume as 
estimated by several independent methods. On the other hand, very slow ageing of this 
polymer was observed. 
All these properties can be explained by the unusual design of the repeat unit of 
PTMSN – bulky Si(CH 
3
) 
3
groups attached directly to the very stiff main chain. 
PTMSN can be considered as a candidate material for membrane separation of hydro-
carbons, and this research is now in progress.